In the typical conventional incandescent light bulb a resistor which will glow incandescently when served with electricity is encapsulated within an evacuated or noble gas-filled glass envelope. Two leads to opposite ends of the resistor sealingly pierce the glass envelope. Usually the bulb is provided with a fitting which combines a mechanical mounting means for the bulb as well as electrical terminals for the two leads, separated by a body of electrically insulating material. Typically the glowing resistor is provided in the form of one or more very fine filamentary strands of a tough, ductile metal such as tungsten. One or both leads of the incandescent filament may incorporate a fuse which is arranged to interrupt or irreversibly sever the electrical circuit through the bulb should an abnormally high voltage be placed across the bulbs leads. Most frequently on incandescent electric light bulbs meant for household and office interior lighting the single fitting of the bulb takes the form of an externally threaded cylindrical base securely externally mounted to the glass envelope. On such bulbs, usually one resistor lead is terminated to the screw-threaded metal collar which forms the sidewall of the base and the other resistor lead is terminated to a boss that is centrally located on the outer end wall of the base. Dielectric material is provided in an annulus closing the space between the boss and the metal collar at the outer end of the bulb base.
In order to use such a bulb, generally it is necessary to screw its base into a socket which has two corresponding, electrically isolated center and peripheral terminals.
Where two or more such bulbs are to be connected in the same electrical circuit, conventionally it is necessary to wire a corresponding number of sockets into the same circuit, and to screw the base of a respective bulb into each. Although the sockets may be serially wired along the circuit, more typically each is wired across the circuit, particularly so that if the light-producing element of one bulb fails the circuit will not be interrupted as to the remaining bulbs and they will continue to be served with electricity.
Although they appear to be rare in the marketplace, double-ended or bifitted incandescent electric light bulbs are known in the art. Generally such bulbs include two diametrically opposed bases, each provided with a respective single electrical terminal for each glowing resistor. Whereas such bulbs may be connected in plurality, base-to-base in series with or without the use of separable intervening bulb-to-bulb connectors, such bulbs cannot be connected in a circuit in parallel to one another without the use of sockets wired in parallel with one another in an electrical circuit which is disposed externally of the light bulbs.